Hydrostatic positive-displacement machines convert hydraulic power in the form of a product of fluid volumetric flow and pressure into mechanical power in the form of a product of torque and rotational speed, and vice-versa. For the power conversion hydrostatic working chambers of variable volume are required, which are defined by pistons. Here, a piston guided in a cylinder either slides against an inclined plane or is connected to the inclined plane by a ball joint. Both variants rely on supporting the piston against the inclined plane. This principle applies to radial piston machines, for example, to axial piston machines of swashplate design or inclined axis design, to wobble-plate machines and to vane-type machines. A special feature of the latter is that the displacement work is performed not by the stroke of the piston in its cylinder, but by the variation in volume of a positive-displacement chamber, which extends radially between a cylinder drum and an eccentric outer ring, and circumferentially between two pistons guided in the cylinder drum. In radial piston machines the inclined plane is formed by a lifting cam, or more precisely lifting face, along which the piston slides, which is associated with periodic working strokes. An axial piston machine of swashplate design comprises a rotating cylinder drum, in the cylinder bores of which working pistons are received, which on the other side are supported so that they slide on a swashplate. In the case of the axial piston machine of swashplate design said working pistons are connected by a ball joint to an end flange of a drive shaft set towards the axis of rotation of the cylinder drum, so that a rotationally fixed connection is produced between the cylinder drum and the inclined axis.
In each case, because of the inclined plane the pistons and the cylinders must absorb both the axial force acting in the direction of the piston longitudinal axis and a lateral force acting transversely or radially in relation to the piston longitudinal axis. This leads to heavy stressing of the circumferential surfaces of the piston and the associated cylinder, particularly in an area where the piston emerges from the cylinder, and in an area of the end portion of the piston permanently guided in the cylinder. Here high solid contact pressures occur between the tribologically paired piston and cylinder. Said extreme areas of the bearing contact are also referred to as guide runouts.
Such high solid contact pressures can lead to a high degree of wear and power losses. If measures such as high-grade materials and/or heat treatment and coating, for example, are taken to counter the wear, the resulting costs of the positive-displacement machine are high. If, on the other hand, an extended bearing contact, that is to say a larger guide length, is chosen in order reduce the solid contact pressures, this takes up more overall space.
Previously known solutions are guide runouts of the pistons or cylinders which are of rigid or solid design in the area of their runout edges. Disadvantages associated with this are a high weight, greater overall space and possibly increased costs.
In order to reduce the solid contact pressure at the guide runouts, the patent specification DE 10 2006 014 222 B4 proposes a working piston which changes from a cylindrical shape to a spherical shape according to the degree of heating. For this purpose, the working piston comprises a hollow internal space into which an expansion element having a higher coefficient of thermal expansion than the working piston is fitted. If the working piston heats up in operation due to friction, the expansion element expands and presses the outer circumferential surface of the working piston into a convexly spherical shape. In this way the stress loading in the area of the guide runouts is shifted away from a high surface unit pressure towards a greater contact area and a lower surface unit pressure. Disadvantages to this are the high jig/fixture and production engineering costs for the working piston, together with a relatively difficult design of the expansion element and difficulty in matching it to the working piston. Furthermore, this solution works satisfactorily only within a narrow operating range, since the resulting spherical convexity is able to compensate for deformation due to lateral forces only at the design temperature.
Patent specification DE 196 10 595 C1 proposes the application of convex chamfers or radiuses to the piston in the area of the guide runouts, which in principle takes up the idea of spherical convexity. These chamfers or radiuses have a constantly varying radius of curvature, which under the effect of the lateral force discussed leads to a more extensive contact of the tribological pairing in the area of the guide runouts. A disadvantage to this is that in producing the outer circumferential surface of the working piston it is necessary to depart from the easily produced, cylindrical shape, which incurs an increased production cost. Moreover, this type of spherical convexity is “fixed”, regardless of the lateral force actually acting, so that this solution also is capable of optimally reducing the surface unit pressure only within narrow operating ranges.